The present invention relates to a wide-band network for matching a signal-conducting line having a characteristic impedance of 50 ohms to a piezoelectric transmitter having a capacitance C.sub.o and a reactance X.sub.o for a center frequency f.sub.o of the frequency band to be transmitted.
Piezoelectric transmitters of this type are used especially in acoustical microscopy for converting high-frequency oscillations into ultrasonic waves and back again. They are mounted in direct acoustic contact on a second propagation medium, for example sapphire. Normally, they consist of a sputtered zinc oxide layer (ZnO) which is embedded between two metal layers, one of which is sputtered onto the sapphire. If then an electric field of a suitable frequency is built up between the metal layers, the ZnO layer generates an acoustic field. The thickness of the layer must be selected in accordance with the working frequency desired for the acoustical microscopy. Typical working frequencies are between 100 MHz and 2 GHz. The thickness of the ZnO layer is selected to be between .lambda./4 and .lambda./2 of the wavelength of the working frequency within the ZnO layer.
As far as the electric connection is concerned, such a transmitter essentially represents a capacitive impedance. Its equivalent circuit can be represented as a resistance R.sub..OMEGA. in series with a capacitance Chd o. Typical values for the resistance are about 1/10 of the reactance of the capacitor at the center of the frequency band to be transmitted if the Q of the transmitter is 10, the quality factor Q being defined as capacitor reactance divided by the resistance.
Direct coupling of the transmitter to a 50-ohm system would bring with it large power losses because of the large electric mismatch. For this reason, electric matching networks are used between the transmitter and the 50-ohm system which are usually optimized for a certain working frequency so that the matching applies only to a small range around this frequency. Therefore, the matching networks have only a limited bandwidth.
For applications covering a large range of bandwidths, it is desired to optimize the matching network over a larger range. This is generally successful only to the extent to which the number of matching elements is also increased. A theoretical upper limit for expanding the bandwidth is essentially determined by the Q of the transmitter.
The practical possibility of implementing the network is made more and more difficult as the number of components increases. For this reason, the effort is made for practical reasons to limit the number of matching elements.
For application in the field of acoustical microscopy, it must also be noted that the matching network is necessarily a part of the acoustical less arrangement. For image scanning, this lens must be moved in an oscillatory fashion. Thus, a need exists for miniaturizing the matching network so that the masses to be moved can be kept as small as possible so as to expend as little energy as possible.